This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2001-70296 filed on Mar. 13, 2001, and No. 2001-399978 filed on Dec. 28, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a digital-analog converting circuit, a display device and a digital-analog signals.
2. Related Background Art
A liquid crystal display has a pixel array part arranged in a matrix form, and drive circuits for driving signal lines and scanning lines. Conventionally, because the pixel array part and the drive circuits have been formed on separate substrates different from each other, it was impossible to reduce cost of the entire liquid crystal display.
Recently, a fabrication technique in which TFTs (Thin Film Transistors) are formed on a glass substrate by using poly-silicon as a material has been developed. If such a technique is applied, it may be technically possible to form the pixel array part and the drive circuit on the same substrate.
An analog pixel voltage has to be supplied to signal lines of the pixel array part. However, the drive circuits are composed of digital components such as gate circuits and flipflops, and perform various signal processings by using the digital signals. Therefore, it is general that a digital-analog converting circuit is provided in the drive circuits and the analog signal converted by the digital-analog converting circuit is applied to the pixel array part.
However, it is currently difficult to form poly-silicon TFTs having uniform properties at high efficiency on the glass substrate. The transistor properties such as a threshold voltage and a mobility may fluctuate largely and the operation speed may become late.
FIG. 9 is a circuit diagram of the conventional digital-analog converting circuit composed of the poly-silicon TFT on the glass substrate, and shows an example of converting the digital pixel data of 3 bits into the analog voltage. The digital analog converting circuit of FIG. 9 has three sets of switching circuits (SW11, SW12), (SW13, SW14) and (SW15, SW16) provided in accordance with the respective bits of the digital pixel data, switching circuits SW17, SW18 and SW19 connected to these circuits, respectively, capacitors C11, C12 and C13 and switching circuits SW20, SW21 and SW22 connected to these switching circuits SW17, SW18 and SW19, a switching circuit SW23 for selecting either of a first voltage V1 or a second voltage V2, a capacitor C14 connected to the switching circuit SW23, an amplifier 2 connected one end of the capacitor C14, a switching circuit SW7 connected to output end of the amplifier 2, and a capacitor C15 for accumulating electric charge passing through the switching circuit SW7 in accordance with the output voltage of the amplifier 2.
FIG. 10 is a diagram showing voltage waveforms of the respective parts in the circuit of FIG. 9 when the digital signals (1, 1, 1) of three bits are inputted. Hereinafter, the operation of the circuit of FIG. 9 will be described with reference to FIG. 10. The first voltage V1 is in 1 volt, and the second voltage V2 is in 4 voltage.
First of all, during time period T1-T2, the switching circuits SW17-SW19 turn on. Therefore, one ends (a, b, c) of the capacitors C11-C13 become 4 volt. Subsequently, during time period T2-T3, the switching circuit SW20 turns on. Therefore, the electric charge moves from the capacitor C11 to the capacitor C14, and one ends (a, d) of the capacitors C11 and C14 become the same voltage (2.5V).
Subsequently, during time period T3-T4, the switching circuit SW21 turns on. Therefore, the electric charge moves from the capacitor C12 to the capacitor C14, and one ends b, d of the capacitors C12 and C14 become the same voltage (3.25V).
Subsequently, during time period T4-T5, the switching circuit SW22 turns on. Therefore, the electric charge moves from the capacitor C13 to the capacitor C14, and one ends (c, d) of the capacitors C13 and C14 become the same voltage (3.625V). At this time, because the switching circuit SW7 also turns on, the electric charge in accordance with the voltage at d point is accumulated into the capacitor C15 via the amplifier 2 and the switching circuit SW7. After time T5, the same operation as that of time period T1-T5 is repeated.
The voltage at d point of FIG. 9 changes gradually during time period T1-T5, and becomes a desired analog voltage at last during time period T4-T5. That is, time length that d point is in the desired analog voltage is short. The switching circuit SW7 can turn on only during time period that d point is in the desired voltage, i.e. time period T4-T5. Accordingly, time period that the output of the analog converting circuit is supplied to the signal line is shorten. Therefore, there is a likelihood that the voltage of the signal line does not fully rise or lower, thereby causing uneven brightness and deteriorating display quality.
An object of the present invention is to provide a digital-analog converting circuit, a display device and digital-analog converting method having even brightness and improving display quality.
In order to achieve the foregoing object, a digital-analog converting circuit configured to output a voltage between a first and second voltages, said voltage corresponding to a digital signal of n bits (n is an integer more than two), comprising:
a plurality of first capacitors configured to accumulate electric charges in accordance with value of the respective bits except for the most significant bit of said digital signal;
a second capacitor capable of accumulating the electric charge in accordance with said first voltage;
a third capacitor capable of accumulating the electric charge in accordance with the value of the most significant bit of said digital signal;
a switching circuit connected on a current path between said second and third capacitors; and
a charge control circuit configured to accumulate the electric charges in accordance with the respective bits except for the most significant bit of said digital signal to the respective fist capacitors, and then transfer the electric charges accumulated to the respective first capacitors into said second capacitor, and accumulate the electric charge in accordance with the most significant bit of said digital signal to said third capacitor, and then turn on said switching circuit in order to transfer the electric charge accumulated to said second capacitor into said third capacitor.
Furthermore, a display device, comprising:
a plurality of switching elements arranged near cross sections of signal lines and scanning lines;
a signal line drive circuit configured to drive the signal lines; and
a scanning line drive circuit configured to drive the scanning lines;
wherein said signal line drive circuit has said digital-analog converting circuit according to claim 1 configured to convert a digital signal expressing pixel information into an analog signal;
wherein the output of said digital-analog converting circuit is supplied to the corresponding signal line.